Rapid evaporation ionisation mass spectrometry (“REIMS”) is a relatively new technique that is useful for the analysis of many different samples including the identification of tissue such as food. For example, it is known to use rapid evaporation ionisation mass spectrometry to determine the animal of origin of a food sample and also the pathological state of a tissue sample. It is also known to use rapid evaporation ionisation mass spectrometry to identify microbes, yeast and fungi.
The known approach for analysing bacterial colonies by rapid evaporation ionisation mass spectrometry involves using bipolar electrosurgical forceps and an electrosurgical RF generator. A bacterial colony is scraped from the surface of an agar layer using the bipolar electrosurgical forceps and a short burst of RF voltage from the electrosurgical RF generator is applied between the bipolar electrosurgical forceps. For example, it is known to apply 60 W of power in a bipolar mode at a frequency of 470 kHz sinusoid. The RF voltage which is applied to the electrosurgical forceps has the result of rapidly heating the particular portion of the bacterial colony which is being analysed due to its nonzero impedance. The rapid heating of the microbial mass results in an aerosol being generated. The aerosol is transferred directly into a mass spectrometer and the aerosol sample may then be analysed by the mass spectrometer. It is known for the control system of the mass spectrometer to utilise multivariate statistical analysis in order to help distinguish and identify different samples.
Rapid evaporation ionisation mass spectrometry is, therefore, a form of mass spectrometry that uses high frequency energy to ablate or vaporise a sample wherein the resulting vapour or aerosol is then subjected to mass spectrometry.
Conventional rapid evaporation ionisation mass spectrometry analysis involves ensuring that the sample is in direct electrical (and physical) contact with a RF voltage supply. This approach works well for tissue identification either ex vivo or in vivo. However, such an approach is problematic if it is desired to process a sample which is housed in a container. In particular, the known approach is problematic if it is desired to process a bacterial culture grown on agar in a petri dish.
It is also problematic to attempt to use the known arrangement for the analysis of tissue sections mounted on a glass slide when it is desired to analyse the tissue sections optically and also to subject the tissue sections to REIMS microprobe imaging.
For completeness, it should be mentioned that other forms of analysis are known such as Matrix Assisted Laser Desorption Ionisation (“MALDI”) analysis. However, such approaches are quite different to REIMS and involve looking at the protein/peptide fingerprint of a sample. This is a relatively slow process that requires significant sample preparation and hence such a process is problematic.
N. Strittmatter, M. Rebec, E. Jones, O. Golf, A. Abdolrasouli, J. Balog, V. Behrends, K. Veselkov, Z. Takats “Characterization and Identification of Clinically Relevant Microorganisms Using Rapid Evaporative Ionization Mass Spectrometry” Anal. Chem. 2014, 86, 6555-6562 discloses a known arrangement wherein two hand-held electrodes in the form of a forceps are used to scrape microbial biomass off from an agar surface. The two electrodes are then squeezed together so as to pinch the biomass between the tips of the forceps. RF power is then applied to the biomass and an aerosol containing analytes is passed to a mass spectrometer for analysis. Accordingly, this reference discloses effectively resistive heating of a sample wherein current flows from one electrode through the sample to the other electrode. Power is dissipated in the sample via resistive heating.
It is desired to provide an improved method of analysing a sample and in particular an improved method of analysing a sample which may comprise a biological sample which has been grown on a culture medium and which is provided in, for example, a petri dish.